Titanium is the preferred structural material for manufacturing marine equipment. The application of titanium in ships and marine engineering fully demonstrates its excellent comprehensive performance. Titanium is used in marine valves to successfully improve the quality of valves and improve product performance.
Titanium corrosion form
Titanium is a highly chemically active metal that exhibits particularly excellent corrosion resistance to most corrosive media. There are several special forms of corrosion of titanium metal, such as high temperature corrosion, stress corrosion [i], pitting corrosion [ii], crevice corrosion, galvanic corrosion [iii] and corrosion fatigue, etc. Titanium is mainly used in marine valves as galvanic corrosion corrosion.
Galva nic corrosion is when two different metals form a galvanic couple in an electrolyte, the corrosion of the anode metal can be accelerated. The amount of corrosion depends on the electrode potential difference between the materials, as well as the area ratio of the anode and cathode and the polarization [iv] characteristics of the two. Titanium is different from ordinary materials in that it is in a passive state in many media and shows a potential similar to that of a passive 18-8 stainless steel.
The potential of titanium in seawater is -0.10V (saturated calomel electrode [v]). When titanium contacts aluminum, zinc, and carbon steel, metals such as aluminum and zinc are corroded. However, their corrosion rate is lower than that caused by contact with stainless steel.
In addition, titanium has a strong polarization tendency, which means that the galvanic couple containing titanium as a cathode generates only a small current. Due to the presence of an oxide film on the titanium surface, the corrosion rate usually does not increase when in contact with other metals. Therefore, other materials on titanium valves for ships should pay attention to the comparison of galvanic corrosion.
Titanium valve structure characteristics [1-5]
- (1) The valve body circulation of the titanium alloy valve adopts the form shown in the figure below (take the stop valve as an example) to reduce the direct erosion of the medium on the sealing surface and the eddy current zone caused by the sharp contraction and expansion of the channel.
- (2) Titanium is more expensive. For economic and reasonable use of titanium, parts that do not come into contact with corrosive media should not be made of titanium.
- (3) The mechanical properties of titanium materials change with temperature, and its strength index decreases with increasing temperature. For example, the tensile strength [vi] and yield strength [vii] at 250 ～ 300 ℃ are about 50% at room temperature. Therefore, even when the design temperature is not high, it should be selected according to the strength value at the design temperature. Titanium has a high yield strength ratio and good endurance strength. Therefore, when the design temperature is below 316 ℃, the tensile strength value at that temperature is often determined by the design strength index. Industrial pure titanium cannot increase the strength index value through heat treatment. Titanium has a small coefficient of thermal expansion. When titanium is used in combination with other materials, attention should be paid to the stress caused by the difference in expansion.
- (4) The plastic deformation range of industrial pure titanium and α titanium alloy during pressure processing is small, and it is easy to crack. Increasing the deformation speed or lowering the deformation temperature may cause cracking during processing. Therefore, it is best not to design cold-worked parts with large deformation. Where the flanging is required, the bending radius should be made as large as possible to prevent the titanium tube from bursting.
- (5) When selecting titanium bolts, it is necessary to consider the problem of stress relaxation due to the creep phenomenon of titanium at normal temperature [viii]. It is best not to use titanium bolts as mandatory sealing connectors. When titanium bolts are required for corrosion resistance, the structural design needs to consider that it is easy to tighten the bolts regularly to ensure sealing. Titanium, like stainless steel, also has frictional bonding and biting. Titanium threads are easy to bite, which can be solved by using different materials or larger clearance threads or with appropriate lubricants. Internal threads that require tapping should be used sparingly, and as far as possible, turn-thread structures with retreating slots should be used.
- (6) Due to the high chemical activity and special physical and mechanical properties of titanium, general cutting methods can be used for cutting, but compared with other commonly used metals, there are also special requirements, master certain processing technology, processing Pay attention during the process. In order to prevent excessive temperature rise, a lower cutting speed should be used. Generally speaking, the cutting speed should be 25% to 50% or lower than that of steel parts with the same hardness. With a large feed rate, the feed rate has little effect on temperature rise.
Do not stop the tool during the cutting process, otherwise it will cause work hardening or sintering, squeeze cracking and damage the tool. The cutting edge of the tool should be sharp, otherwise the tool will easily wear, because the wear of the cutting tool is not proportional to the amount of cutting, and the wear is very small at the beginning. Once the tool becomes blunt, the wear will increase immediately. Use enough cutting fluid (5% sodium chloride aqueous solution, oil-water volume ratio of 1:20 emulsified oil and other lubricating cooling fluid) to lubricate and cool, reduce the heat on the blade, and wash away the chips.
The development of petroleum, chemical, aerospace, shipbuilding and marine fields has promoted the rapid development of the valve industry, and has conducted in-depth development and research on the structure, material and production process of titanium alloy valves, thereby improving the reliability and The life of pipeline control equipment meets the requirements of valve technology development in various fields. At present, the flange titanium alloy globe valve has passed the appraisal of the relevant authoritative organizations, and it has been successfully applied to the new generation of marine and marine engineering key projects. [6]
Note:
- The failure phenomenon of stress corrosion materials, mechanical parts or components under the combined action of static stress (mainly tensile stress) and corrosion.
- Pitting corrosion refers to short-term pitting corrosion, which refers to a very localized corrosion pattern that forms small holes on the metal surface in a corrosive medium, also known as pitting corrosion. Corroded pores exist in isolation, some are tightly together.
- Due to different corrosion potentials, dissimilar metals are in contact with each other or connected by other conductors, in the same medium, causing local corrosion of the dissimilar metals contact parts.
- Polarization refers to the phenomenon that things polarize under certain conditions, causing their properties to deviate from their original state.
- Saturated calomel electrode is a prop invented using the principle of electrochemistry. The calomel electrode with saturated potassium chloride solution as electrolyte is developed with the electrode potential of 0.2412V at 25 ℃. It acts as a salt bridge in chemical production.
- Tensile strength (tensile strength) is the critical value of the transition of the metal from uniform plastic deformation to local concentrated plastic deformation, and is also the maximum load capacity of the metal under static tensile conditions.
- Yield strength is the yield limit when a metal material yields, that is, the stress that resists a small amount of plastic deformation.
- Creep: The phenomenon that the strain of a solid material increases with time while the stress is kept constant.
